The invention generally relates to methods of reducing bit errors in digital communication systems. More specifically, the invention relates to methods for reducing bit error in digital communication systems such as that disclosed by German patent DE-PS 38 21 871 C2, the teachings of which are fully incorporated herein by reference.
Analog and digital transmission methods employed on lines and cables for data transmission are known to be susceptible to disturbing external influences, for example, electromagnetic field influences, that can modify data or information being transmitted thereover. For example, one or more bits of digital data or information can be "turned around" or otherwise altered during transmission. These alterations of the bits are known as bit errors.
Because international long distance information transmission systems are particularly susceptible to bit errors, international standardization committees, such as, for example, the CCITT have established minimum rules for bit error rates.
Generally, these systems include transmission links, every transmission link being composed of two parts:
(A) one part which is a path and relates to the transmission of the information over a physical distance between two nodes. These paths increasingly are being realized with optical communications cables. PA0 (B) another part is a switching network which relates to the through-connection of the information in the respective nodes wherein communications systems are installed.
Optical transmission links are far less susceptible to the above mentioned external influences, so that transmission reliability is extremely high given the employment of optical transmission links. As a result, by employing optical transmission systems, potential disturbing influences have been decisively reduced in the transmission parts of these systems. However, by contrast thereto, bit errors frequently occur during the through-switching events in the switching networks of these communications systems.
Due to their central significance for the network nodes, these switching networks are usually implemented in levels such as pairs or triplets, this giving rise to a redundancy which is used to assure continued connection through a network. Given a malfunction of one switching network level, this makes it possible to through-connect a bit stream over another switching network level. This is particularly true given a total outage of a switching network level.
However, bit falsifications during the through-connection of a bit stream which, for example, are produced by aging individual components can be problematical. Such errors are difficult to analyze and eliminate since they have no influence whatsoever on the function of the switching network as a through-switching unit.
Error recognition methods for recognizing and correcting bit errors in switching networks are known and are, for example, realized in the form of methods for monitoring of check sum information co-transmitted with the data words. German published application DE-OS 24 27 668, having corresponding United Kingdom patent application GB 1 447 713, the teachings of which are fully incorporated herein by reference, discloses such a method wherein an additional parity bit is attached to each data or information word in each channel before the actual through-switching event in the switching network, an information word generally comprised of bits.
In general, a parity bit represents the binary check sum of the individual bits of the corresponding data information word or inversion thereof. A parity bit is a self-checking code that is attached to a set of data bits (a word); the parity bit being set so that the sum of the one's bits is always odd (odd parity) or even (even parity). After the through-switching event, the binary check sum of the through-connected word is again calculated and is compared to the transmitted parity bit. When the co-transmitted and the newly calculated parity bits deviate from one another, there is a transmission error recognized that is retained in a corresponding table memory.
As a result, statistical statements about the condition of the corresponding switching network levels can be made over specific time spans. With these a switching network level error rate in the past preferably can be employed for through-connection of the bit streams. A comparable method is also disclosed by German patent DE-PS 38 21 871 C2, the teachings of which are incorporated herein by reference.
One problem that arises in these methods, however, is that the falsification of an even number of bits in an information word is not recognized as a bit error since the parity calculation at the output side of a switching network level leads to the same result as the parity calculated at the input side. Although statistical statements about the quality of a through-connection of the data or information words can be made given recognized bit errors, the bit error remains uncorrected and is thus forwarded to the destination subscriber in erroneous form. This procedure harbors serious deficiencies insofar as additional bit errors can be incurred and add up in further network nodes.